Optical proximity sensors are widely used in products such as cell phones and smartphones for detecting proximity to a human head. These sensors often use infra-red (IR) light emitting diodes (LEDs) and optical sensors like photodiodes. The sensor detects the amount of IR light that is reflected from the proximity target, e.g. as the ear area of a human head as a phone is brought into close proximity to the sensor. Proximity detection can be used to trigger response in the phone such as turning off the cell phone screen, in order to conserve cell phone battery power.
A problem, however, can occur when the cell phone is pressed hard against the head of a person who has very dark and dense hair. In such case the hair absorbs most of the incident IR light emitted from the LED, and the small amount of IR light that is reflected eventually has no optical path back to the detector. Although black hair does reflect some IR light, optical apertures in the cover glass—typically used to protect proximity sensors—are physically separated. When a dark object such as black hair is pressed tightly against the glass, the dark object may obstruct the light path, blocking any reflected IR light from reaching the aperture over the sensor. In addition to black hair, similar problems can occur when the phone is placed against the side of the head of someone wearing a stocking cap or other object that can occlude any reflected IR light from reaching the detector. With very dense opaque objects, the problem can also occur with light coloured objects, since they can prevent reflected light passing through the LED aperture, from reaching the sensor aperture, by virtue of being opaque.
The result is that the sensor detects no proximity signal, and electronics in the phone erroneously conclude that there is no nearby proximity target. In response the cell phone turns the screen back on, wasting precious battery energy, while the cell phone user continues using his phone.
In most cell phone designs (due to design reasons) the IR LED and optical sensor are required to reside behind the cell phone's cover glass, in order to render these optical parts invisible. On the other side part of the design of a cell phone proximity sensor must address the issue of optical crosstalk, which occurs when light reflects from the inside and/or outside surfaces of the cell phone's cover glass due to Fresnel reflection at the glass surfaces. The same measures, however, used to minimize optical crosstalk typically give rise to the inability to detect dark objects placed on or nearly against the glass.
A common approach on improving black hair response is to design the optics such that the critical point lies just outside of the glass outer surface. The critical point characterizes the point where proximity detection first occurs as one proceeds along the z direction, i.e. the axis perpendicular to the glass surface. This occurs when the emission cone of the LED first intersects with the detection cone or field of view of the detector. A shortcoming of this approach is that often the critical point is far from being an infinitesimally small point, and is generally distributed in space, as the emission/detection cones seldom go to zero abruptly at their outer extents. A consequence is that placing the critical point just outside the outer glass surface, so as to detect black hair lying against the outer surface, generally results in some proximity detection of the glass outer surface (being ˜4% reflective) but resulting in significant undesirable optical crosstalk. An additional concern relates to manufacturing tolerances, whereby variations in sensor to glass or LED to glass spacing, or part to part variations in LEDS as the LED divergence angle can often vary significantly with low cost LEDs and may result in the critical point lying inside the glass, such that there can be significant yield losses in cell phones due to increased optical crosstalk.
Because of the above described problems in optical proximity sensor design, industry has developed standards to be passed by a given sensor arrangement. For example, a sensor is required to pass a black card test. A sensor arrangement passes the black card test if it produces a proximity signal under standardized conditions. The test involves a card of defined low reflectivity which simulates dark objects and is positioned on top of the cover at zero distance.
In conclusion, there is a need in the art to provide an optical proximity sensor that allows for improved detection of objects placed close against its cover, yet allows suppressing of optical crosstalk.